Compositions of bonding materials based on aqueous alkali metal silicate solutions have been known for a long time, and are widely used. Such bonding material compositions can be used for the manufacture of mineral insulating materials, impregnating materials, coating materials, paints, and putty, as well as for bonding wood, paper, ceramics, and mineral materials. The known alkali metal silicate solutions, in particular sodium silicate solutions, will usually have a molar ratio of SiO.sub.2 :Me.sub.2 O (Me=sodium and/or potassium) in a range of 2.0:1 to 3.4:1, and a solids content of 35 to 50 weight percent.
Such bonding materials can be used for the manufacture of molds for metal castings. The aqueous alkali metal silicate solutions, which preferably have a molar ratio of SiO.sub.2 :Me.sub.2 O in the range of 2.0:1 to 2.9:1, are strengthened with a fine particle filler such as sand; and the resulting mixture is then formed into the desired shape, and strengthened with a suitable hardener. A good hardener for such purpose is carbon dioxide gas.
Disadvantages of such bonding material compositions, including the process used, are the reduced flowability of the mixture which may prevent a pore-free filling of complicated hollow shapes, and the reduced initial strength of the gas treated cores within the first hour which may result in difficulties in their transportion. Furthermore, the completed molds and cores are very hard to break after the metal has been cast. In order to improve the "knock-out" of the molds after casting, various carbohydrates such as sugar, molasses, starch or cellulose derivatives are added to the bonding materials. However, none of these additives have a salutory effect on the initial strength of the gas treated molds.
For example, German Patent Application DE-OS No. 27 50 294 discloses as bonding materials an aqueous solution containing alkali metal silicate and a carbohydrate from the group of glycanes, glycanoligosaccharides, and glycanitol derivatives of the oligosaccharides, the monosaccharides, and the disaccharides, and their derivatives, as well as an oxyanion of boron, tin, germanium, tellurium, or arsenic which can form a water-soluble complex with the carbohydrate.
In PCT Patent applications Nos. WO 80/01 767 and WO 80/01 768, compositions of bonding materials are described that are based on an aqueous alkali metal silicate solution containing water-soluble carbohydrates from the groups of mono-, di- and polysaccharides or their derivatives as well as urea (No. WO 80/01 767) and phenolic compounds, selected from the group of compounds with one, two, or three phenol groups, polynuclear hydrocarbons with phenol groups, and phenolic acids (No. WO 80/01 768).
The subject matter of German Patent Application DE-OS No. 28 56 267 relates to non-self-curing bonding materials to be used for the manufacture of foundry molds and cores, and available as aqueous solutions of alkali metal silicates with the addition of one or several anionic and/or nonionic surfactants. The latter additives enhance the flowability of the bonding materials.
U.S. Pat. No. 3,032,426 to R. S. Lee discloses the use of an ethanolamine, preferably diethanolamine, as a cure accelerator in a process for producing foundry cores and molds from alkali metal silicate-sand compositions. In the Lee process, an alkali metal silicate is mixed with sand and then an ethanolamine is added to the silicate-sand mixture. Curing is carried out with carbon dioxide.